Production of single crystal semiconductor material

ABSTRACT

THE PERFECTION OF SINGLE CRYSTAL SEMICONDUCTOR MATERIAL IS MAINTAINED WITHIN A BODY GROWN BY THE VAPOR PHASE DEPOSITION OF THE SEMICONDUCTOR MATERIAL ON A HEATED SINGLE CYRSTAL SEED FILAMENT BY ALLOWING SUBSTANTIALLY FRICTION-FREE LATERIALF AND LONGITUDINAL MOVEMENT OF THE FILAMENT AS IT IS HEATED PRIOR TO THE VAPOR DEPOSITION, ALLOWING SUBSTANTIALLY FRICTION-FREE LONGITUDINAL MOVEMENT OF THE BODY AS IT CONTRACTS WHILE COOLING AFTER THE DEPOSITION PROCESS, AND MAINTAINING THE DIFFERENCE IN TEMPERATURE THROUGHOUT THE CROSS SECTION OF THE FILAMENT AND THE GROWING BODY AT A VALUE TO PREVENT SUBSTANTIAL DENSI-   TIES OF DISLOCATIONS FROM FORMING IN THE CRYSTAL STRUCTURE DURING THE ENTIRE PROCESS.

I March 7, 1972 L. D. DYER 3,647,561

PRODUCTION OF SINGLE CRYSTAL SEMICONDUCTOR MATERIAL Filed Dec. 31, 1969 Ill" l3 lllllll ll T H 42b 44 3 (y) E -420 22 I 2/ 40 INJEHFOR.

39 LAWRENCE 0. DYER 1 United States Patent 3,647,561 PRODUCTION OF SINGLE CRYSTAL SEMICONDUCTOR MATERIAL Lawrence D. Dyer, Richardson, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex. Filed Dec. 31, 1969, Ser. No. 889,573 Int. Cl. B011 17/00 U.S. Cl. 148-1.6 6 Claims ABSTRACT OF THE DISCLOSURE Ihe perfection of single crystal semiconductor material is maintained within a body grown by the vapor phase deposition of the semiconductor material on a heated single crystal seed filament by allowing substantially friction-free lateral and longitudinal movement of the filament as it is heated prior to the vapor deposition, allowing substantially friction-free longitudinal movement of the body as it contracts while cooling after the deposition process, and maintaining the diiierence in temperature throughout the cross section of the filament and the growing body at a value to prevent substantial densities of dislocations from forming in the crystal structure during the entire process.

This invention relates to the production of semiconductor bodies. In another aspect, this invention relates to a method and apparatus for controlling the stress on a seed filament of semiconductor material during operations involving the vapor phase deposition of semiconductor material upon the filament. In still another aspect, this invention relates to a method of maintaining the perfection of single crystal semiconductor material grown by vapor deposition of the semiconductor material onto a single crystal filament.

Single crystal semiconductor material, such as single crystal silicon or germanium is commonly used in the manufacture of semiconductor devices, such as diodes, transistors, and integrated circuits. The semiconducting material used in these applications must be free from defects which destroy the desired electrical properites of the product.

One method of producing semiconductor materials suitable for the manufacture of electric components involves the vapor deposition of the semiconductor material upon a single crystal starting filament of the same material. During the deposition, the starting filament grows radially until a single crystal body of the desired size is formed. The single crystal body is then cut into semiconductor slices which are treated by various techniques to produce diodes, transistors, integrated circuits, and the like.

A conventional method for producing single crystal silicon, for example, is to suspend an elongated generally cylindrical or flat sided filament within a reactor such as a quartz tube which it fitted with suitable end plates and graphite electrodes (chucks) within which the ends of the starting filament are clamped. The filament is then heated by developing a potential across the graphite electrodes and thereby passing current therethrough.

During most operations, the filament is initially heated at elevated temperatures and treated with vapors such as hydrogen and/or hydrogen halide to precondition the surface of the filament by etching. Next, the temperature of the filament is lowered somewhat, and a gaseous stream of hydrogen and a silicon halide is passed over the lilament. The gaseous components in the stream react upon contacting the hot starting filament and thereby deposit silicon on the surface of the filament. Conventional procedures for this type are disclosed in U.S. 3,168,422 and U.S. 3,172,791.

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When practicing the above described process in conventional equipment, problems have arisen which result in the non-uniform growth and/or defects in the grown semiconductor material due to (1) stresses which arise from sideward restraint and non-axial positioning of the upper and lower chucks; (2) stresses which occur due to longitudinal expansion of the filament as it is heated from ambient temperature to the etching and deposition temperature, and stresses occurring due to longitudinal contraction of the filament as it is cooled from deposition temperature to ambient temperature; and (3) stresses which are caused by radial temperature gradients which are formed in the filament and growing semiconductor body. These gradients are generally caused by either a steady state temperature difference that conventionally exists between the rod surface and the rod interior, which temperature difference increases as the rod grows, and/or the temperature gradient that comes into being when the rod is cooled at the end of the run.

The most common defects caused by the three abovedescribed types of stresses are crystal dislocations formed in the growth of single crystal material used in semiconductor devices. These dislocations are a known source of degradation of semiconductor devices.

One object of this invention is to provide an improved method and apparatus for producing semiconductor bodies.

Another object of this invention is to provide a method and apparatus for preventing any unwanted stresses from developing in a semiconductor seed filament or growing crystalline body during a vapor phase deposition process for producing semiconductor bodies.

Still a further object of this invention is to provide an improved method and apparatus for producing dislocation-free single crystalline semiconductor bodies.

According to the invention an improved method and apparatus are provided for growing a single crystal semiconductor body wherein an elongated single crystalline starting filament is coupled between a pair of electrode chucks within a vapor deposition zone and heated and contacted with reacting vapors therein to cause deposition of single crystal semiconductor material thereon while allowing substantially friction-free lateral and longitudinal movement of said filament due to thermal expansion and contraction thereof and maintaining the temperature gradients of radial cross sections of the growing semiconductor body at values so that stress gradients induced thereby result in substantially no dislocation formation.

This invention can be more easily understood from a study of the drawing which is a perspective view partially in section of an apparatus of this invention.

Now referring to the drawing, reactor 10 comprises a cylindrical quartz reactor tube which is held between end plates 12 and 13 by ring clamps 14 and 15, respectively. End plates 12 and 13 are secured to ring clamps 14 and 15 by nut and bolt assemblies 16. Single crystal semiconductor filament 17 is positioned within reactor 10 and held in electrical communication between graphite chucks 18 and 19, as will be described below. Electrode 19a extends through end plate 13 and connects to graphite chuck 19. Electrical coupling device 20 connects between graphite chuck 18 and electrode clamp 21, carrying lead 22. Electrode 19a and lead 22 connect to a conventional electric power source.

Conduit 23 extends through end plate 13 and serves to introduce reactant gases to the interior of reactor 10. Conduit 24 extends through end plate 12 and functions to remove by-product gases and unreacted reactants from the interior of reactor 10.

Electrical coupling device comprises the improved electrical coupling device disclosed in copending application Ser. No. 783,376 filed Dec. 12, 1968, which issued as US. Pat. 3,538,483 on Nov. 3, 1970. Electrical coupling device 20 functions to relieve longitudinal stresses on filament 17 during the heatup and cooldown operation. An alternative electrical coupling device is disclosed in my copending application Ser. No. 744,028, filed on July 11, 1968, which issued as US. Pat. 3,558,281 on Jan. 26, 1971.

Electrical coupling device 20 generally comprises a bottom section and a top section which is movable in a vertical relationship to the bottom section. The top section of electrical coupling device 20 comprises gas shield 25, chuck 18, and rod 26 which extends downwardly from the inner face of gas shield into socket chamber 28 of the bottom section of coupling device 20.

The bottom section of coupling device 20' is carried by end plate 12 and generally comprises a tubular housing 27 which encloses cooling chamber 29 and a socket chamber 28 which is adapted to receive rod 26. The bottom of socket chamber 28 is closed by plug 30.

Plug 30 is generally a hollow cylindrical plug which is closed at its lower end and threaded at its upper end and thereby adapted to threadably engage screw threads positioned in the lower end of socket chamber 28. Plug 30 functions to retain a conductive fluid such as a molten or liquid metal, for example, mercury and preferably gallium. Heating device 32 is attached to the bottom of plug 30 for the purpose of supplying sufficient heat to maintain metal 31 in the liquid state. Leads 32a are connected to a conventional power source. Electrode clamp 21 is positioned around plug 30 as described above.

Porous bushing 33, which is generally a gas permeable cylindrical member, is positioned adjacent the open end of socket chamber 28. Bushing 33 can be any tubular porous metal known in the art, such as a porous bronze prelubricated bushing which has been heated to remove the oil therefrom. Porous bushing 33 is suspended in the open end of socket 28 by holding members 34 which engage and seat with indentations around either end of bushing 33 to yield an annular space 35 between the wall of socket chamber 28 and the outside periphery of porous bushing 33.

Control gas inlet. conduit 36 extends through housing 27, cooling chamber 29 and the upper portion of socket chamber 28 to communicate with annular space 35. Control valve 37 is operatively positioned within control gas inlet conduit 36. Control gas outlet conduit 38 communicates between the interior of socket chamber 28 below porous bushing 33 and pressure control valve 39. Conduit 40 is positioned within conduit 38 at a point upstream of control valve 39, and is operatively connected to pressure gauge 41. Cooling chamber 29 is connected to coolant inlet and outlet conduits (not shown).

As shown in the drawing, silicon segment 42 is positioned within graphite chuck 18. Silicon segment 42 comprises a shank portion 42a adapted to fit within chuck 18 and a pointed portion42b adapted to contact the lower end of filament 17. Pointed portion 42b is sharpened by a suitable grinding or milling operation to a point having a breadth of from 1 micron to about inch in diameter. As will be explained in detail below, the use of silicon segment 42 will relieve any lateral stresses in the filament due to non-axial alignment of chucks 18 and 19 as the filament is heated prior to the vapor phase deposition process. Other alternative lateral stress relieving methods and devices can be utilized in the scope of this invention. Such suitable techniques are disclosed in my copending patent application Ser. No. 876,243, filed Nov. 13, 1969.

The basic cylindrical reactor illustrated in the drawing can be used for the vapor phase deposition of semiconductor materials known in the art such as, for example, silicon, germanium, and compounds in Groups III-A and V-A of the Periodic Table as illustrated on page 4 13-2 of the Handbook of Chemistry and Physics, Chemical Rubber Company (1964). However, for the purposes of illustration, this invention will be described in relation to the production of a single crystal silicon rod.

As illustrated, reflective shield 43 is positioned within reactor tube 11 such that it completely encircles filament 17. The interior surface of reflective shield 43 is a highly polished or mirror surface adapted to reflect thermal energy emitted from filament 17 and the resultant growing semiconductor crystalline body. The internal reflecting surface of reflective shield 43 can be a mirror deposited from a solution, or from vacuum evaporation of a reflective substance, which is non-deleterious to the reaction, for example, silver. Alternatively, the internal reflectvie surface of reflective shield 43 can be silver foil, or a highly polished stainless steel surface. It is noted that reflective shield 43 can be positioned around the exterior of quartz reactor tube 11 within the scope of this invention; however, it is preferred that a smaller diameter shield 43 be positioned as illustrated in the drawing. Reflective shield 43 carries apertures 44 so that the temperature of filament 17 can be monitored by conventional means such as an optical pyrometer.

The reflective shield 43 functions to eliminate physical stresses caused by radial temperature gradients in filament 17 and the growing semiconductor rod. Specifically, as the rod grows during the deposition process, thermal energy is constantly being emitted from the surface thereof. Therefore, in order to maintain the surface of the rod at the proper deposition temperature, current flow through the rod is increased, thereby heating the interior of the rod. In conventional operations, the rate of conduction of thermal energy from the interior to the exterior of the rod is slower than the rate of energy lost from the surface of the rod by radiation and convection, and the temperature difference between the interior and exterior of the rod increases as the radius of the rod increases. Similarly, during cooldown, the rods exterior surface can cool very rapidly due to radiation and convection, but the rod mterior can cool only at a slower rate by a conductive transport of heat to the rods surface. The action of reflective shield 43 will eliminate or substantially reduce these unwanted temperature gradients which cause unwanted stress in the rod from the action of the thermal coeflicient of expansion of the material.

In operation of the apparatus as illustrated in the drawmg, filament 17 is initially suspended from graphite chuck 19 and silicon segment 42 is operatively positioned within graphite chuck 18. Next, electrical coupling device 20 is actuated by initially supplying current through leads 32a to heating device 32 to thereby cause gallium 31 to liquefy. When heating device 32 has melted gallium 31 and rod 26 is free to move in the pool of liquid metal, valves 37 and 39 are opened to allow control gas to pass through the porous bushing 33 and out both from under gas shield 25 and conduit 38. A suitable control gas can be any gas which is not deleterious to the etching procedure and noncontaminating to the subsequent deposition procedure. Preferably, the control gas is hydrogen. When pressure valve 39 is closed, an increased pressure on gallium 31 results, which in turn will force rod 26 upward. This in turn will cause chuck 18 carrying silicon segment 42 to be moved upward until the tip of pointed portion 42b is in contact with the lower end of filament 17 as illustrated in the drawing.

Current is passed through filament 17 and segment 42 thereby heating the tip of pointed portion 42b and causing softening and melting thereof. This melting will allow deformation to relieve any stresses in filament 17. The melting of the tip of pointed portion 42b will create molten silicon which will spread over the surface of the end of seed filament 17. This action will create surface tension between melted silicon and the end of filament 17 to aid in holding filament 17 and segment 42 in intimate contact during the etching and deposition operation.

The longitudinal stresses within filament 17 are next relieved prior to etching. Valve 39 is adjusted until the pressure read by gauge 41 will be suificient to support the weight of the upper section of electrical coupling device 20 comprising rod 26, gas shield 25 and chuck 18, and onehalf the weight of filament 17 while providing a sufiicient fiow of gas to hold rod 26 in a spaced relationship from the inside surface of porous bushing 30. This off-setting pressure will allow filament 17 to expand uniformly during heating without unnecessary distortive forces acting thereon. During this operation, cooling water can be passed through cooling chamber 29.

After the initial heating procedure, the flow of hydrogen gas through electrical coupling device 20 can be reduced by partially closing valve 37. This action will stop the gas bearing action of electrical coupling device 20 while maintaining the gas barrier by maintaining the pressure within conduit 38 at a value to offset the weight of the top section of electrical coupling device 20 and approximately one-half the weight of filament 17. Thus, since filament 17 has already expanded, it is not necessary that frictionless electrical contact be made with lead 22 at this time.

After the stresses are relieved from filament 17, the vapor phase etching and deposition processes are carried out in a conventional manner. For example, filament 17 is initially heated to a temperature of about 1200 C. and a mixture of hydrogen and hydrogen chloride (about wt. percent hydrogen chloride in the hydrogen) is passed through the interior of reactor 10 via conduits 23 and 24 for a period such as one-half hour. Next, suitable reactants are passed to the interior of the reactor such as for example, trichlorosilane or silicon tetrachloride, hydrogen chloride, and hydrogen to contact filament 17 and deposit pure silicon on the surface thereof.

During the vapor phase deposition operation, thermal energy radiated from starting filament 17 is reflected by the reflective surface on the interior of reflective shield 43. This reflected energy will assure that the temperature of the surface of filament 17 will remain sufliciently high to prevent a temperature differential between the interior and exterior of filament 17 which will cause unwanted physical stress due to the coeflicient of thermal expansion of silicon and thereby cause undesirable dislocations Within the single crystalline body.

The temperature differential between the interior and exterior surface of the growing single crystal semiconductor rods should be such that the resultant stress gradient induced in each radial cross section of the rod by the ter-mal coeflicient of expansion will be less than of the upper yield stress (as determined by ASTME 8-66) of a corresponding single crystal material which has substantially no dislocations therewithin (less than about 500 per square cm.). For example, when growing single crystal silicon, the temperature differential between the interior and exterior of the growing silicon rod should not exceed about 60 C. once the interior temperature of the rod has reached 900 C. Most preferably, the temperature throughout each radial cross section of the growing semiconductor rod should be substantially the same.

The reaction is continued until the resulting single crystal semiconductor rod has reached the desired diameter. Just prior to the end of the deposition procedure when current to filament 17 is stopped, valve 37 is opened further to produce again the gas bearing action of electrical coupling device 20. Pressure within outlet conduit 38 is maintained at a value which will olfset the weight of the top section of electrical coupling device and approximately one-half of the weight of the grown silicon rod. Current flow through filament 17 is then shut off and filament 17 is allowed to cool and contract with substantially no distortive forces acting thereon. The reflective action of reflective shield 43 will insure that the dilferential temperature between the interior and exterior of the rod will not exceed the gradient defined above.

This invention is further illustrated by the following example:

EXAMPLE Using a reactor as illustrated in the drawing, an 8" x A" single crystal silicon filament 17 is suspended from graphite chuck 19. Next, a silicon segment approximately A1 in diameter and 1" long having a cylindrical chuck portion and a sharply pointed end portion is positioned in graphite chuck 18 so that the point thereof is directed toward the lower end of the filament suspended from chuck 19. Reflective shield 43 is made of a silver foil, and is approximately 10" long and surrounds the entire length of the filament. Valves 37 and 39 are next actuated to allow electrical coupling device 20 to move upward and the point of the segment to contact the lower end of filament 17. Hydrogen is introduced through inlet conduit 23 and exhausted through outlet conduit 24. The reactor is flushed and current is passed through electrodes 18 and 19. Pressure valve 39 is adjusted sothat electrical coupling device 20 supports the weight of rod 26, shield 25, chuck 18, the pointed segment, and one-half the Weight of the filament. The pointed end of silicon segment melts when the filament reaches 600 to thereby allow deformation of the coupling segment and the resultant release of lateral stress within seed filament 17. Next, the current is increased through the filament thereby raising it to a temperature of about 1325 C., while hydrogen at the rate of about 10 milliliters circulates through reactor 10. This etching procedure is continued for 11 minutes.

Valve 37 is partially closed to stop the gas bearing action of electrical coupling device 20 while maintaining the gas barrier and maintaining the pressure within conduit 38 at a value to oifset the weight of the top section of electrical coupling device 20 and approximately /2 the weight of filament 17.

After this time, two liters per minute of hydrogen chloride are introduced into the 10 liter per minute hydrogen stream to form a mixture which is circulated over filament 17 for 1 minute, after which the hydrogen chloride flow is terminated. Ten liters per minute of hydrogen is permitted to continue to flow over the filament for an additional two minutes, at the end of which, 1 liter of hydrogen chloride is introduced into the hydrogen stream, bringing the total gas flow to 11 liters per minute. This gaseous mixture is passed over the filament for a period of 15 minutes. Next, the hydrogen chloride flow is terminated for a period of 1 minute during which 10 liters per minute of hydrogen continues to circulate over the filament. After reactor 10 is purged with pure hydrogen, 2 liters per minute of hydrogen chloride are again introduced into the stream and the mixture is allowed to flow over filament 17 for 1 minute to complete etching procedure.

After the etching procedure, trichlorosilane is introduced into the stream and the flow of hydrogen and hydrogen chloride controlled until the total feed stream is 30 liters per minute containing 4.5 mole percent of trichlorosilane, 0.5 mole percent hydrogen chloride, and mole percent hydrogen. The current flow through filament 17 is adjusted so that temperature during deposition is maintained at about 1250 C. This process is continued for about 20 hours until the resulting silicon rod has reached a diamenter of about /2".

Next, valve 37 is opened further to produce again the gas bearing action of electrical coupling device 20- Pressure within outlet conduit 38 is maintained at a value which will offset the weight of the top section of electrical coupling device 20 and approximately one-half the weight of the grown silicon rod. Current through the rod is now shut off and it is allowed to cool and contract in the reactor. At this time, the flow of trichlorosilane and hydrogen chloride is also stopped and the flowing hydrogen is allowed to purge the reactor for 5 minutes. Next, the hydrogen flow is stopped to leave a blanket of hydrogen around the formed rod as the rod slowly cools to room temperature.

This technique produces a grown single crystal silicon rod with substantially no dislocations therein (less than about 500 per squire cm.).

The silcon rod produced by the process of this invention such as set forth in the above example will process substantially no dislocations which are induced from physical restraints on the rods because both the lateral and longitudinal stresses on the rod are relieved. In addition, substantially no dislocations are introduced within the filament during the initial heatup from room temperature to the etching temperature because the shield will reflect emitted energy back to the filament, thereby maintaining the differential temperature between the interior and exterior of the filament below that which will result in substantial formation of dislocations. Additionally, substantially no dislocations are formed during the deposition procedure as the rod grows and after the deposition procedure as the rod cools because of the above described action of the reflective shield.

While this invention has been described in relation to its preferred embodiments, it is to be understood that various modifications thereof will now be apparent to those skilled in the art upon reading the disclosure and it is intended to cover such modifications as fall within the scope of the appended claims.

I claim:

1. In a method of growing a single crystal semiconductor body by the chemical vapor deposition of semiconductor material onto a single crystal starting filament maintained at an elevated temperature, wherein said filament is vertically suspended between upper and lower support members adapted to supply an electric current for heating said filament, the improvement comprising:

supporting said lower support member on a body of conductive fluid for the purpose of providing substantially friction-free lateral and longitudinal movement of said filament; passing an electric current through said filament to achieve reaction temperature; and maintaining a temperature diiTerence no greater than 60 C. between the interior and the exterior of the growing single crystal semiconductor body at said reaction temperature to prevent the formation of substantial dislocation densities therein. 2. A method as defined by claim 1 wherein said conductive fluid is a liquid and further including the step of controlling the buoyancy of said body of liquid by the application of a variable gas pressure to the surface thereof.

3. A method as defined by claim 1 wherein said lower support member includes a pointed semiconductor member positioned such that the point thereof provides the sole contact with the lower end of said filament.

4. The method of claim 1 wherein said temperature diiference is maintained at a value such that the maximum stress induced thereby due to the thermal coeflicient of the expansion of the single crystal material is less than 15% of the upper yield stress of the single crystalline material which is substantially free of dislocations.

5. The method of claim 1 wherein said semiconductor material is silicon.

6. The method of claim 1 wherein the temperature throughout each radial cross section of said growing single crystal semiconductor body is substantially equal.

References Cited UNITED STATES PATENTS 3,188,182 6/1965 Morelock 1481.6 1,728,814 9/1929 Van Liempt 148-1.6 1,733,752 10/1929 Ramage 148--1.6 3,226,270 12/1965 Miederer et a1. -u l481.6 3,222,217 12/1965 Grabmaier 148174 3,328,199 6/1967 Sirtl 148174 DELBERT E. GANTZ, Primary Examiner G. J. CRASANAKIS, Assistant Examiner 

